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Author Topic: How is pressure affected by gas reactions?  (Read 963 times)

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Gokijet

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How is pressure affected by gas reactions?
« on: January 15, 2012, 09:36:15 AM »

When gases react within a definite space(constant volume), will the pressure of the gases change if the reaction had released/absorbed heat? (question1)
Our teacher told us that the pressure will be lower after a reaction, because there is less moles in the product than in the regents, but then where is the law of conversion of mass?(question2) And about the temperature part, he didn't mention it at all, which is why I posted this.


Any response will be appreciated, thanks!
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Arkcon

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Re: How is pressure affected by gas reactions?
« Reply #1 on: January 15, 2012, 09:53:03 AM »

You have more than two questions, mixed up in bits and pieces across your missive.  You also mention moles of product before and after the reaction -- that is not a constant for all gas reactions, you may sometimes have or less.  So you should also review the information you have and see what the real question is.  Also, you bring up the conservations of mass.  Good for you for remembering that.  However, the question is about pressure, and that is not tied to mass.
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UG

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Re: How is pressure affected by gas reactions?
« Reply #2 on: January 15, 2012, 09:54:52 AM »

When gases react within a definite space(constant volume), will the pressure of the gases change if the reaction had released/absorbed heat? (question1)
Yes, if the volume is kept constant and the temperature of the gas inside the space increases/decreases, the pressure of the gas will change. You might have seen the ideal gas law? PV=nRT, where the product of pressure (P) and volume (V) is equal to the number of moles of gas (n) multiplied by a constant R and the temperature of the gas (T). This equation indicates the pressure of the gas is proportional to the temperature (for constant volumes)

Our teacher told us that the pressure will be lower after a reaction, because there is less moles in the product than in the regents, but then where is the law of conversion of mass?(question2)
Was he/she talking about a particular reaction? In some reactions there will be more moles of gaseous products than reactants but the reverse can also occur. The law of conservation of mass is not broken, gases simply occupy more volume that liquids and solids. 1 kg of liquid water occupies a lot less room than 1 kg of gaseous water.
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Gokijet

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Re: How is pressure affected by gas reactions?
« Reply #3 on: January 15, 2012, 10:13:32 AM »

Much thanks for your responses! I am sorry for the misleading title and the screwed up paragraph structures, thanks for your time Arkcon.
Also thanks to UG for the answers, that cleared up my confusion. Our teacher was talking about a particular reaction, too bad for me I didn't remember it...but I did learned from you that there can be more or less moles of products, it's totally new for me(I guess I had missed it in regular chem). For the first question, if that is the case, then I'll have to ask my teacher about what happened with that questions. Thanks for you time too! My questions are solved!
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